Traditional instruments for measuring the optical properties of liquid samples employ sample holders such as cuvettes, and the measurements are made on the bulk properties of the liquid.
WO 2007131945 discloses a microvolume analyser employing a drophead having a surface which is adapted to receive a drop of liquid to be tested, the drophead being positioned in use relative to a source and a detector to illuminate a drop received thereon so that the drop causes an interaction in the path of the electromagnetic radiation between the source and detector. Unlike with bulk systems, the surface of the drophead is dimensioned to constrain the drop to adopt a shape which is dominated more by surface tension forces than by gravitational forces.
Liquid drops with such small volumes cause particular considerations which do not exist for bulk volume analysers. The properties of the drop are dependent on the drop shape which is in turn dictated by the volume of liquid in the drop due to the dominance of surface tension forces over gravity. As a result, any inaccuracy in the drop volume leads to inaccuracy in measurement. A particular source of variation in drop volume is a evaporation from the drop between the time when the drop is deposited on the drophead and the time when the measurement is taken. There can be a great deal of variation in this regard, particularly if there are repeated measurements or where different operators take different measurements.